The present invention relates to an alkaline electrolyte for galvanic elements, which electrolyte additionally contains materials with gelling or adsorption capabilities.
The field of application of the present invention primarily extends to alkaline primary and secondary elements, since their operational reliability makes fixation of the electrolyte desirable. However, the present invention is not limited to specific battery systems of the alkaline type, but may also encompass other systems, such as fuel cells.
Ordinarily, such elements operate with an alkaline electrolyte based on the KOH/H.sub.2 O system. However, in H.sub.2 /air fuel cells, or metal/air elements, there is an unavoidable introduction of CO.sub.2 from ambient air which rapidly creates a KOH/K.sub.2 CO.sub.3 /KHCO.sub.3 /H.sub.2 O electrolyte system. Consequently, in alkaline systems, precautions must normally be taken to exclude air (CO.sub.2).
Acidic electrolytes such as H.sub.2 SO.sub.4 /H.sub.2 O can be immobilized by means of gellable materials such as SiO.sub.2, as well as by Al.sub.2 O.sub.3, TiO.sub.2 or zeolites. However, these inorganic substances cannot be used to fixate alkaline electrolytes because such substances exhibit both basic and acidic properties, making it impossible to preclude them from dissolving in the alkaline medium. For example, in the case of TiO.sub.2, the literature sometimes speaks in favor of (cf. Gmelins Handbook of Inorganic Chemistry (8th Edition), Verlag Chemie, Weinheim, Germany 1951, Chapter 41, page 255) and sometimes against (cf. Cotton-Wilkinson, Inorganic Chemistry, Weinheim, Germany 1985, page 709) stability in aqueous alkaline hydroxides.
As long as the system is not limited to a quantity of potassium or sodium hydroxide solution which is just sufficient for operation of the storage battery, and which can be fixated through capillary action only within the pore structure of the electrodes and the separator, fixation of the electrolyte in alkaline cells has always been achieved by means of gellable organic substances. Practical examples are carboxymethyl cellulose, potato starch, alginates, lignin, soluble resins and polyvinyl alcohol derivatives or compounds which yield a polyvinyl alcohol through hydrolysis or saponification.
When using such thickening agents, the danger arises that during periods of disuse or in the course of discharging of the cell, the liquid electrolyte will separate from the gel. Large quantities of gelating medium can sometimes stop this process. However, according to U.S. Pat. No. 4,332,870, this is also the cause of a reduction in the ion conductivity of the electrolyte, i.e., an increase in its ion resistance. As a remedy, it is proposed to simultaneously add a multivalent alcohol (through which, homogenizing of the electrolyte is to be achieved) to the gelating medium, in this case a starch product with a hydrophilic side chain. In so doing, the highly water soluble alcohol functions as a binder between the powdery anode metal (e.g., zinc) and the gelating medium, and to gel the latter by absorption of water, in situ, so that the metal particles are uniformly distributed in the gel which is formed.
German Patent Publication (DE-OS) 2,736,578 teaches that by drying a coprecipitate of Co(OH).sub.2, Cd(OH).sub.2 and Mg(OH).sub.2, a gellable powder is yielded which can be used as an additive and which can be utilized in the electrolyte of an alkaline zinc secondary cell as a source of cobalt and cadmium ions. The purpose is to cause the coprecipitation of these metals to interface with the deposition of zinc at preferred growth loci of its crystal lattice, and thereby prevent dendrite growth. In so doing, the magnesium hydroxide performs only the function of a carrier or an extender.